1. Field of the Invention
The present invention relates to improvements in the lifetimes of carbon-supported electrodes used in fuel cells and batteries by the removal of low-valent sulfur (which acts as a catalytic poison) from the carbon in these electrodes, through a catalytic process. The invention also relates to removing sulfur from coal and other carbon blacks. The invention also relates to producing sulfate ion.
2. Description of the Related Art
As used herein, "low-valent sulfur" refers to sulfur in a zero or negative oxidation state. This includes, but is not limited to, zero-valent organic sulfur (sulfur covalently bonded to carbon, but not to oxygen). Such organic sulfur may be part of an organic ring or chain (typically 5- or 6-membered heterocyclic rings). Most typically, organic sulfur will be part of an aromatic group like thiophene or thiophene derivatives. Low-valent sulfur also includes negative valence species such as metal sulfides (e.g., CoS, FeS, FeS.sub.2), and hydrogen sulfide H.sub.2 S. These species may be found in carbon samples as impurities, where they may be dissolved, agglomerated, adsorbed, etc. As used herein, the term "sulfur-containing carbon" refers to all of these species.
Referring to FIG. 8, fuel cells 50 are devices that produce electricity from electrochemical reactions. The reactants in fuel cells are replenished as needed, typically in a continuous flow process. Consequently, fuel cells 50 generally have one or more inlet ports 52 for reactants (i.e., the fuel and the oxidant), and an outlet port 54 for the reaction products, in the reaction vessel 56. All fuel cells have, in the reaction vessel, an anode 58 and a cathode 60, which are separated from each other by an electrolyte 59. In the fuel cell art, the term "electrolyte" is used in its broadest sense: a material, composition, or structure that passes ions but not electrons between the two electrodes. Most modern fuel cell electrolytes are solid phase, such as polymer membranes, although liquid electrolytes for fuel cells are also known. Some exemplary solid phase electrolytes include sulfonated fluorocarbon polymers such as Nafion.TM. (DuPont), and ceramic oxides such as yttria-stabilized zirconia (Y.sub.2 O.sub.3.ZrO.sub.2, a.k.a. YSZ).
The electrodes used for fuel cells typically use some type of noble metal or noble metal alloy as a catalyst. These are referred to in the art as electrocatalysts. Common electrocatalysts include platinum metal (e.g., platinum black), platinum alloys (such as PtSn, PtRu, and multiple ternary platinum alloys), and cobalt-centered macrocycles. For a variety of reasons, in particular the high cost of many of these electrocatalysts, it is typical to put the electrocatalyst on a carbon matrix. This can be done by mixing the electrocatalyst with a carbon matrix, or supporting the electrocatalyst on a carbon support matrix. A particularly common electrode, ca. 25 .ANG. Pt metal clusters on a vulcan carbon support matrix is described in greater detail below. Some types of carbon that have been used as matrices for electrocatalysts include vulcan carbon, pyrolitic carbon, glassy carbon, and carbon blacks. Vulcan carbon is a form of carbon that contains low-valent sulfur, due to its production process. It has a number of desirable properties, but, as described below, it also has some undesirable properties for use as an electrocatalyst matrix.
Proton-exchange membrane fuel cells (PEMFCs) are an attractive alternative to petroleum-based energy sources. In these electrochemical cells, hydrogen is oxidized to protons at the anode; the protons then transfer through the PEM and combine with reduced oxygen at the cathode to form water. See generally A. J. Appleby and F. R. Foulkes, Fuel Cell Handbook, (Van Nostrand Reinhold, New York) (1989) and H. P. Dhar, J. Electroanal. Chem., 357, 237 (1993), both of which are incorporated by reference herein. These oxidation and reduction reactions are catalyzed at the fuel-cell electrodes by platinum or noble-metal alloys. To reduce the cost of the electrodes, the traditional Pt-black electrodes of PEMFCs and phosphoric acid fuel cells (PAFCs) have been replaced by carbon-supported noble-metal clusters. By using clusters on the order of 25 .ANG. in diameter instead of noble metal blacks, the mass activity of the noble metal is dramatically increased. See K. Kinoshita, J. Electrochem. Soc., 137, 845 (1990).
A chronic concern in the engineering of a fuel cell is to prevent poisoning of the electrode catalysts, particularly by sulfur. One source of sulfur is H.sub.2 S gas, a common impurity in fuel gas. The sulfurous gas dissociatively adsorbs on the Pt surface and then blocks the available sites for catalysis of the fuel. See D-T. Chin and P. D. Howard, J. Electrochem. Soc., 133, 2447 (1986), and J. Biswas et al. Catal. Rev.--Sci. Eng. 30, 161 (1988), both of which are incorporated by reference herein.
Another source of sulfur in fuel cell electrodes comes from the carbon support. In the vulcanized carbon that is favored as a support, to its high electronic conductivity (especially relative to other carbon blacks) and surface properties (e.g., wetting behavior) (see J. McBreen et al., J. Appl. Electrochem., 11, 787 (1981), incorporated by reference herein). However, there is approximately 5000 ppm (weight) of sulfur in vulcan carbon. See K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties (John Wiley and Sons, New York) (1988), G. Tamizhmani and G. A. Capuano, J. Electrochem. Soc., 141, L132 (1994), both of which are incorporated by reference herein. This corresponds to one sulfur atom for every 3.3 platinum atoms in a typical 10 wt % Pt/carbon electrocatalyst (see Kinoshita et al., J. Electrochem. Soc. 137 845 (1990), incorporated by reference herein), so the ratio of Pt surface atoms to S atoms is ca. 1.5 to 1 in a 10 wt % Pt/carbon material. Because a single sulfur atom can poison multiple Pt sites (see J. Biswas et al., supra, and T. D. Halachev and E. Ruckenstein, Surf. Sci., 108, 292 (1981), both of which are incorporated by reference herein), there is sufficient S in the vulcan carbon to poison the entire surface area of the supported Pt clusters. Yet, the excellent initial performance of PAFCs and PEMFCs and the low overpotentials observed at their anodes suggest that the sulfur in the carbon support has no influence on the fuel cells, and this high concentration of native sulfur is generally overlooked. Until now, it had not been shown that carbon-associated sulfur in fuel cell electrodes would affect the performance of fuel cell electrocatalysts. Currently available Pt-C electrodes lose 10-100% of their performance efficiency after about 6 months of use.
Referring to FIG. 9, batteries are conceptually and structurally similar to fuel cells, except that they are closed systems--fuel normally is neither added to nor removed from batteries during discharge or optional recharge (for rechargeable batteries). Thus, a battery 62 has an anode 64 and a cathode 66 separated by an electrolyte 65. Similar concerns for electrode lifetime apply to batteries as apply to fuel cells. One immediate problem is that many batteries lose ca. 10% of their total battery capacity after the first charge-discharge cycle, whereupon reasonably stable performance results. Slightly different terminologies are used for batteries, however. Instead of "electrocatalyst", practitioners in the battery art refer to "active material". Also, the roles of the anode and the cathode are reversed during recharge.
A chronic barrier to the use of naturally occurring hydrocarbons (i.e., coal and crude petroleum) as an energy source is the presence of environmentally unfriendly sulfur in these substances. Techniques are available for removing sulfur from naturally occurring hydrocarbons, including thiophene-like sulfur or S that is covalently bonded to carbon atoms. However, many of these procedures require elevated temperatures (&gt;100.degree. C.), or long times to achieve desulfurization. One proposed method (see S. B. Lalvani et al., Fuel Processing Tech. 21 117-123 (1989)) performs this process electrochemically, but teaches that a highly concentrated electrolyte solution (1 M NaOH) is required. It is generally undesirable to carry out industrial processes in electrolytes of high ionic strength (e.g., concentration .gtoreq.1 M of acid, base, or salt solutions), for reasons including cost, workplace safety, and pollution control.
Sulfuric acid, an important commercial reagent, is commonly manufactured by hydrogenation of sulfate (SO.sub.4.sup.2-) ion. Accordingly, commercial processes for the production of sulfate are desired.